Trailers have been around for many years, yet every summer and winter one can observe the owners of boats and snowmobiles, respectively, backing up those devices on trailers with great difficulty. The problem arises from the fact that a trailer being backed-up is an inherently unstable system. A trailer being pushed wants to turn around and be pulled (i.e., to jackknife) instead. To compensate for this instability, the driver must skillfully alternate the direction of his steering so as to cause the trailer to want to turn around and be pulled from opposite sides thereby repeatedly crossing the centerline of the pushing vehicle. Various innovations have been introduced to address this problem in whole or in part. Prior art reveals several attempts to address the problems associated with backing a trailer. The simplest solutions address parts of the problem ranging from ways of sensing the angle of the hitch (see: Kollitz, U.S. Pat. No. 4,122,390), to sensing and displaying the angle of the hitch (see: Gavit, U.S. Pat. No. 3,833,928), to sounding an alarm when a jackknife condition exists or is imminent (see: Kimmel, U.S. Pat. No. 4,040,006). While these solutions are helpful, they only each address a part of the backing problem. Shepard in his U.S. Pat. No. 7,715,953 (the '953 patent) teaches a complete working system. However, in that teaching, some new needs arise that are addressed by the teaching of the present invention, such as how to install a complete working system as an after market product that can easily be installed or removed. The present invention also teaches a sensor for measuring the angle formed between the centerline of the vehicle and the centerline of the trailer (i.e., the hitch angle sensor or, as it is sometimes also known, the articulation angle sensor) such that it does not interfere or collide with the trailer tongue or any other parts of the hitching system that can easily be installed or removed. In particular, an angle sensor is needed that can get its measurement in-line with the axis of rotation of the trailer tongue upon the hitch ball (i.e., to measure the articulation angle of this hitch joint) without actually being located at that axis of rotation. This hitch angle sensor, in particular, must be designed not to be damaged either during hitching up a trailer (due to a collision between the hitch and sensor with a part of the trailer) nor while towing on the highway (due to kicked up debris). The present invention also teaches a steering sensor that does not interfere with the driver's ability to steer and that also can easily be installed or removed.
Trailer guidance systems such as the system disclosed in the '953 patent require sensors for detecting the hitch angle and the turning radius and output means for displaying the intended trailer destination. However, most vehicles do not have integral turning sensors and most trailers and/or hitches do not have integral hitch angle sensors. In particular, the turning radius must typically be calculated from a measurement of a portion of the steering mechanism of the vehicle thereby requiring a sensor to take that measurement. One way to determine the turning radius (as is disclosed in the '953 patent) is to sense the angular deflection to the front wheels of the vehicle away from the straight line position (i.e., the steering angle) and use that angular value (along with the vehicle's wheel base) to compute the turning radius. Another way to determine the turning radius (as disclosed herein) is to take a measurement of the steering mechanism by sensing the angular position of the steering wheel, compare that measurement to a reference position (such as the steering wheel angle when the vehicle is steered to travel in a straight line), compute the difference between these two angles, translate the difference into an angular deflection to the wheels of the vehicle away from the straight line position (i.e., the steering angle), and (again, as is disclosed in the '953 patent) compute the turning radius. Likewise, a hitch sensor must measure the angle formed between the centerline of the vehicle and the centerline of the trailer (i.e., the hitch angle).